This patent application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-164034 filed on Jul. 10, 2009, the entire contents of which are incorporated herein by reference.
The embodiments discussed herein are related to a printed circuit board unit and an electronic device.
A printed circuit board unit is formed by mounting electronic components on a printed circuit board. Examples of electronic components are an integrated circuit, a resistor, and a capacitor. In recent years, integrated circuits have become increasingly multi-function and high performance, and therefore a large number of components are being mounted on a printed circuit board. Furthermore, electronic devices with such a built-in printed circuit board unit are becoming increasingly compact. Therefore, there is demand for compact printed circuit board units. Accordingly, components are being increasingly densely mounted in the printed circuit board unit. Hence, it is becoming difficult to secure enough space on the printed circuit board for mounting multiple components. One approach is to provide a printed circuit board unit in which electronic components may be mounted not only on the front side but also on the back side (see, for example, Japanese Laid-Open Patent Application No. 2000-150775).
The components mounted on the front side 2A of the printed circuit board 2 and the components mounted on the back side 2B of the printed circuit board 2 are electrically connected to each other by through holes 9 formed in the printed circuit board 2. A method other than using through holes 9 may be applied to electrically connect the components mounted on the front side 2A of the printed circuit board 2 and the components mounted on the back side 2B of the printed circuit board 2. For example, a built-up substrate may be used as the printed circuit board 2, so that the components are electrically connected to each other by interlayer wiring and via holes that are formed in the built-up substrate.
However, the operation of forming interlayer wiring and via holes entails complex procedures and increased cost. Therefore, through holes are preferably used for connecting the components in terms of simplifying the manufacturing process and reducing cost.
The through holes 9 are arranged in a grid array in the printed circuit board 2. Furthermore, the through holes 9 are densely formed in the printed circuit board 2 in a region where the BGA type integrated circuit 5 is provided.
In
Therefore, in a conventional structure for mounting components, this high density formation region A becomes a so called dead space that may not be used for mounting components. Hence, in a conventional structure, it is difficult to densely mount components on the printed circuit board.
According to an aspect of the invention, a printed circuit board unit includes a printed circuit board including through holes arranged in a grid array on which an integrated circuit is mounted; and a flexible substrate provided on a back side of the printed circuit board in such a manner as to cover the through holes, wherein first lands to which the integrated circuit is connected are formed on a front side of the printed circuit board, the first lands being connected to first ends of the through holes, second lands to which the flexible substrate is connected are formed on the back side of the printed circuit board, the second lands being connected to second ends of the through holes, third lands are formed on a front side of the flexible substrate so as to face the second lands of the printed circuit board, and fourth lands are formed on a back side of the flexible substrate, the fourth lands being electrically connected to the third lands.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.
A description is given, with reference to the accompanying drawings, of embodiments of the present invention.
The printed circuit board unit 10A includes a printed circuit board 11, BGA type integrated circuits 12, BGA type memories 13, bypass capacitors 14, a flexible substrate 15, and SOP type transformers 16. In the printed circuit board unit 10A according to the present embodiment, electronic components on the front side and electronic components on the back side are electrically connected by through holes 20 that are arranged in a grid array.
The BGA type integrated circuit 12 is for performing various control processes in a server. A BGA (Ball Grid Array) package having a surface-mount structure is applied as the BGA type integrated circuit 12. Solder balls 17 are used as electrodes in the BGA type integrated circuit 12. The solder balls 17 are connected to first lands 36 (see
The first lands 36 are connected to the through holes 20 formed in the printed circuit board 11. The through holes 20 are formed by plating the insides of through holes that are formed so as to pierce through the printed circuit board 11. The first lands 36 are formed at the top ends of the through holes 20. Furthermore, electrode pads are formed at the bottom ends of the through holes 20. The electrode pads are for connecting the printed circuit board 11 to the flexible substrate 15.
The through holes 20 are arranged in a grid array in the region where the BGA type integrated circuit 12 is mounted. Hereinafter, the region where the through holes 20 are formed and the BGA type integrated circuit 12 is mounted is referred to as the high density formation region A. Furthermore, the electrode pads formed in the high density formation region A are referred to as second lands 37.
In the present embodiment, four BGA type integrated circuits 12 are mounted on the printed circuit board 11, as illustrated in
The BGA type memory 13 is, for example, a buffer memory. The BGA type memory 13 is typically preferably disposed near the BGA type integrated circuit 12 for the purpose of increasing the processing speed of the BGA type integrated circuit 12. However, the BGA type memory 13 may be affected by heat from the high-heat-generating BGA type integrated circuit 12. In order to prevent this, there are cases where the BGA type memory 13 is provided with radiating fins. However, in the present embodiment, the BGA type memory 13 is electrically connected to the BGA type integrated circuit 12 via the flexible substrate 15. Therefore, the BGA type memory 13 is prevented from being affected by heat from the high-heat-generating BGA type integrated circuit 12, without providing radiating fins. Furthermore, when reworking is performed due to loose connection of the BGA type integrated circuit 12, the entire flexible substrate 15 that is a slave substrate may be removed, so that the BGA type memory 13 is prevented from being heated.
Next, a description is given of the flexible substrate 15 that is a slave substrate, with reference to
The third lands 38, which are for connecting to the printed circuit board 11, are connected to the second lands 37 of the printed circuit board 11 by soldering. Accordingly, the flexible substrate 15 is fixed to the printed circuit board 11.
The fourth lands 39 and 40 for mounting electronic components are formed on the back side 15B of the flexible substrate 15. The BGA type memory 13 is connected to the fourth lands 39 formed in an extending part 48B (described below) of the flexible substrate 15, and the bypass capacitor 14 is connected to the fourth lands 40 that are formed facing the high density formation region A. Wiring patterns 41 are connected to the third lands 38 for connecting to the printed circuit board 11 formed on the front side 15A, and also to the fourth lands 39 and 40 that are formed on the back side 15B. In the present embodiment, an opening 19 is formed on either side of the flexible substrate 15. Some of the third lands 38 are electrically connected to the fourth lands 39 and 40. As illustrated in
The flexible substrate 15 is disposed at a position on the back side 11B of the printed circuit board 11, in such a manner as to correspond to the position where the BGA type integrated circuit 12 is mounted on the other side of the printed circuit board 11.
The size of the flexible substrate 15 is the same as or larger than that of the high density formation region A formed on the printed circuit board 11. In the present embodiment, the size of the flexible substrate 15 as viewed from the top is larger than that of the high density formation region A formed on the printed circuit board 11. Therefore, the flexible substrate 15 is located in a region where the BGA type integrated circuit 12 is mounted on the front side 11A of the printed circuit board 11 (i.e., in the high density formation region A). As described above, in the present embodiment, there are four BGA type integrated circuits 12 provided on the front side 11A of the printed circuit board 11. Accordingly, there are four flexible substrates 15 provided on the back side 11B of the printed circuit board 11, in such a manner as to correspond to the positions where the BGA type integrated circuits 12 are formed.
Accordingly, in the printed circuit board unit 10A to which the mounting structure according to the present embodiment is applied, electronic components such as the bypass capacitor 14 may be mounted in the high density formation region A. As components are densely mounted in the present embodiment, the printed circuit board unit 10A is made compact, thus reducing the size of an electronic device such as a server in which the printed circuit board unit 10A is installed.
Furthermore, the BGA type memory 13 may be mounted at a position corresponding to the position where the BGA type integrated circuit 12 is mounted on the printed circuit board 11. Accordingly, the length of wiring between the BGA type integrated circuit 12 and the BGA type memory 13 is short. Hence, signals are exchanged between the BGA type integrated circuit 12 and the BGA type memory 13 at high speed, thus enhancing the reliability of the printed circuit board unit 10A.
First, a description is given of a reworking method performed on the BGA type integrated circuit 12, with reference to
The top heating cover 26 is connected to a hot air supplying means (not illustrated) that supplies hot air indicated by arrows. The hot air supplied from this hot air supplying means to the top heating cover 26 is hot enough for melting the solder balls 17. As the solder balls 17 melt, the BGA type integrated circuit 12 may be removed from the printed circuit board 11. The transformers 16 mounted adjacent to the BGA type integrated circuit 12 are protected by the heat-proof jigs 28, and are thus prevented from being damaged by the hot air supplied from the hot air supplying means.
The bottom heating cover 27 is for heating the back side 11B of the printed circuit board 11. Similar to the top heating cover 26, hot air indicated by arrows is supplied to the bottom heating cover 27 from a hot air supplying unit. Furthermore, the heating plates 29 are also for heating the back side 11B of the printed circuit board 11. The bottom heating cover 27 and the heating plates 29 are for preheating the printed circuit board 11. By preheating the printed circuit board 11, the inside of the top heating cover 26 may be quickly heated to a temperature high enough for melting the solder balls 17, thereby enhancing the efficiency of rework.
In the printed circuit board unit 10A according to the present embodiment, the BGA type memories 13 are mounted near the outer periphery of the flexible substrate 15 (the extending part 48B described below). These BGA type memories 13 may be damaged due to heat while rework is performed on the BGA type integrated circuit 12.
To prevent such a problem, fixing resin 22 is removed during the rework process. The fixing resin 22 is used for fixing the parts near the outer periphery of the flexible substrate 15 to the back side 11B of the printed circuit board 11. By removing the fixing resin 22, the regions of the flexible substrate 15 where the BGA type memories 13 are mounted are separated from the printed circuit board 11. Furthermore, the BGA type memories 13 are covered with heat proof covers 30. Accordingly, the BGA type memories 13 are prevented from being damaged by heat while rework is performed on the BGA type integrated circuit 12. Hence, the rework process on the BGA type integrated circuit 12 may be reliably performed, even if the flexible substrate 15 is used for enhancing the efficiency in mounting components in the printed circuit board unit 10A.
The BGA type integrated circuit 12 is a semiconductor device for performing various control processes in the server in which the printed circuit board unit 10B is installed. Accordingly, the BGA type integrated circuit 12 performs high-speed processing and generates high heat during operation. Furthermore, a transformer 16A mounted on the front side 11A of the printed circuit board 11 also generates high heat during operation. Hence, the group of high-heat-generating components that generate high heat during operation, such as the BGA type integrated circuit 12 and the transformer 16A, are densely mounted on the front side 11A of the printed circuit board 11.
Conversely, the BGA type memory 13 does not generate high heat during operation like the BGA type integrated circuit 12. Similarly, a bypass capacitor 16B mounted on the back side 11B of the printed circuit board 11 does not generate high heat during operation. The group of low-heat-generating components that do not generate high heat during operation such as the BGA type memories 13 and the bypass capacitor 16B are densely mounted on the back side 11B of the printed circuit board 11 via the flexible substrate 15.
As described above, the group of high-heat-generating components and the group of low-heat-generating components are mounted separately from each other, i.e., on the front side 11A and the back side 11B of the printed circuit board 11, respectively. If these components were mounted on the same side of the printed circuit board 11, components for blocking heat would also need to be provided on the same side. However, in the present modification, there is no need to provide components for blocking heat, thus enhancing the efficiency in mounting components.
The bottom printed circuit board unit 10B includes a group of low-heat-generating components mounted on the top side as viewed in
Furthermore, the BGA type integrated circuits 12 that are high-heat-generating components in the printed circuit board unit 10A and the printed circuit board unit 10B may be provided with radiating fins 33 as illustrated in
Next, a description is given of a method of fabricating the printed circuit board unit 10A according to the first embodiment.
The printed circuit board 11 is a resin substrate including epoxy resin, and is a rigid substrate having a multilayer structure. The printed circuit board 11 is fabricated by a known method. At the position where the BGA type integrated circuit 12 is mounted, multiple through holes 20 are formed so as to correspond to the terminals of the BGA type integrated circuit 12. The through holes 20 are formed by forming through holes in a resin substrate that is the base material, and then plating the insides of the through holes with copper. The first lands 36 are formed at the front ends of the through holes 20. The second lands 37 are formed at the back ends of the through holes for connecting the printed circuit board 11 to the flexible substrate 15. The first lands 36 and the second lands 37 may be formed together with or separately from the process of plating the through holes 20 with copper.
The flexible substrate 15 is fabricated by forming the third lands 38 for connecting the flexible substrate 15 to the printed circuit board 11 and the fourth lands 39 and 40 for mounting electronic components, on a resin film (for example, a polyimide film) having insulating properties and flexibility. The third lands 38 and the fourth lands 39 and 40 are formed by placing copper films on the polyimide film with the use of a printing technology.
Plural flexible substrates 15 are formed from a single polyimide film. Specifically, after forming the third lands 38 and the fourth lands 39 and 40, the polyimide film is cut into separate flexible substrates 15. The openings 19 may be formed at the same time as cutting the polyimide film into separate flexible substrates 15.
After the printed circuit board 11 and the flexible substrate 15 are formed in the above manner, as illustrated in
Next, as illustrated in
After the solder paste 43A and the fixing resin 22 are applied on the back side 11B of the printed circuit board 11 as described above, the flexible substrates 15 are mounted on the back side 11B of the printed circuit board 11. The solder paste 43A contacts the third lands 38 of the flexible substrate 15, and the fixing resin 22 contacts the flexible substrate 15. As the fixing resin 22 has viscosity before being thermally-hardened, the flexible substrates 15 are tentatively fixed to the back side 11B of the printed circuit board 11 due to this viscosity.
Next, a reflow process is performed on the printed circuit board 11 on which the flexible substrates 15 have been tentatively fixed, in order to solder-mount the second lands 37 on the printed circuit board 11 onto the third lands 38 on the flexible substrate 15. Accordingly, the flexible substrate 15 is fixed onto the printed circuit board 11 by solder 43. The fixing resin 22 is also thermally-hardened in response to heat, and therefore the printed circuit board 11 and the flexible substrate 15 adhere to each other by the fixing resin 22.
As described above, the flexible substrates 15 are fixed on the printed circuit board 11 by the solder 43 and the fixing resin 22.
In the present embodiment, a transparent resin film is used as the flexible substrate 15. Therefore, the soldering positions where the solder 43 is applied and the adhering positions where the fixing resin 22 is applied may be directly viewed from the bottom of the printed circuit board 11 through the flexible substrate 15 that is a transparent resin film. Thus, soldering faults with the solder 43 and adhering faults with the fixing resin 22 may be easily and directly detected. Accordingly, the flexible substrate 15 may be connected and fixed to the printed circuit board 11 with high reliability.
In the present embodiment, the solder 43 is used for connecting the second lands 37 of the printed circuit board 11 to the third lands 38 of the flexible substrate 15. However, the method of connecting these substrates is not limited to soldering; a pressure bonding method or an ACF (Anisotropic Conductive Film) connecting method is also applicable.
When the flexible substrate 15 is disposed on the printed circuit board 11, solder paste (not illustrated) is applied to lands other than the second lands 37 that have been formed on the back side 11B of the printed circuit board 11 and to the fourth lands 39 and 40 formed on the back side 15B of the flexible substrate 15. The solder paste may be applied by a screen printing method. The flexible substrate 15 is a thin film (for example, 0.05 mm). Therefore, even if the flexible substrate 15 is disposed on the printed circuit board 11, it is possible to simultaneously perform screen printing on the printed circuit board 11 and screen printing for applying solder paste on the flexible substrate 15.
When the process of printing the solder paste is completed, electronic components such as the BGA type memories 13 and the bypass capacitors 14 are tentatively fixed to lands other than the second lands 37 on the back side 11B of the printed circuit board 11 and the fourth lands 39 and 40 on the back side 15B of the flexible substrate 15. Next, a reflow process is performed on the printed circuit board 11 on which the BGA type memories 13 and the bypass capacitors 14 have been tentatively fixed, in order to solder mount the electronic components such as the BGA type memories 13 and the bypass capacitors 14 onto the printed circuit board 11 and the flexible substrate 15.
Accordingly, electronic components such as the BGA type memories 13 and the bypass capacitors 14 are mounted on the printed circuit board 11 and the flexible substrate 15.
After electronic components have been mounted on the back side 11B of the printed circuit board 11, electronic components are mounted on the front side 11A of the printed circuit board 11. That is to say, solder paste (not illustrated) is applied on the first lands 36 and lands for mounting the transformer 16, which are formed on the front side 11A of the printed circuit board 11. The solder paste may be applied by a screen printing method.
When the process of printing the solder paste is completed, electronic components such as the BGA type integrated circuits 12 and the transformers 16 are tentatively fixed to the first lands 36 and lands other than the first lands 36 on the front side 11A of the printed circuit board 11. Next, a reflow process is performed on the printed circuit board 11 on which the BGA type integrated circuits 12, etc., are tentatively fixed, in order to solder mount the electronic components such as the BGA type integrated circuits 12, etc., onto the printed circuit board 11.
Accordingly, electronic components such as the BGA type integrated circuits 12, etc., are mounted on the printed circuit board 11.
The printed circuit board unit 10A is fabricated by performing the above series of procedures. As the BGA type integrated circuit 12 generates a large amount of heat, radiating fins 45 may be provided on the BGA type integrated circuit (see
Next, a description is given of a modification of the above-described method of fabricating the printed circuit board unit 10A.
In the above-described method of fabricating the printed circuit board unit 10A, after the flexible substrates 15 are disposed on the printed circuit board 11, electronic components such as the BGA type integrated circuit 12 and the BGA type memories 13 are mounted on the printed circuit board 11 and the flexible substrate 15. In the present modification, before disposing the printed circuit boards 11 on the flexible substrate 15, the electronic components are mounted on each of the printed circuit board 11 and the flexible substrate 15. Then, the flexible substrates 15 on which the electronic components have been mounted are disposed on the printed circuit board 11 on which the electronic components have been mounted.
Specifically, on the printed circuit board 11 illustrated in
Next, a reflow process is performed on the printed circuit board 11 on which electronic components such as the BGA type integrated circuits 12 have been tentatively fixed, in order to solder-mount the electronic components such as the BGA type integrated circuits 12 onto the back side 11B of the printed circuit board 11.
Next, a process of mounting electronic components on the flexible substrate 15 is described. This process may be performed in parallel with the process of mounting electronic components on the printed circuit board 11. In order to mount electronic components on the flexible substrate 15, solder paste is applied to the fourth lands 39 and 40 formed on the back side 15B of the flexible substrate 15 as illustrated in
Next, a reflow process is performed on the flexible substrate 15 on which the electronic components such as the BGA type memories 13 and the bypass capacitors 14 have been tentatively fixed, in order to solder-mount the electronic components such as the BGA type memories 13 onto the back side 15B of the flexible substrate 15.
When electronic components have been mounted on the printed circuit board 11 and the flexible substrate 15 in the above-described manner, the solder paste 43A is applied to the positions where the second lands 37 for connecting to the flexible substrate 15 are formed on the back side 11B of the printed circuit board 11. Furthermore, the fixing resin 22 having thermosetting properties is applied to predetermined positions on the back side 11B of the printed circuit board 11 (the solder paste 43A and the fixing resin 22 are not illustrated in
Next, the flexible substrate 15 is tentatively fixed to the back side 11B of the printed circuit board 11. In the tentatively-fixed state, the solder paste 43A is in contact with the third lands 38 on the flexible substrate 15, and the fixing resin 22 is also in contact with the flexible substrate 15.
Next, a reflow process is performed on the printed circuit board 11 on which the flexible substrate 15 is tentatively fixed, in order to solder-mount the second lands 37 of the printed circuit board 11 onto the third lands 38 of the flexible substrate 15. Accordingly, the flexible substrate 15 is solder-fixed onto the printed circuit board 11 by the solder 43. Furthermore, the fixing resin 22 also is thermally-hardened in response to heat, and therefore the flexible substrate 15 is adhered to the printed circuit board 11 by the fixing resin 22.
In the present modification, the printed circuit board unit 10A is fabricated by performing the above procedures.
However, according to the present modification, when the electronic components such as the BGA type integrated circuits 12 are mounted on the printed circuit board 11, the electronic components such as the BGA type integrated circuits 12 may be tested. Likewise, when electronic components such as the BGA type memories 13 are mounted on the flexible substrate 15, the electronic components such as the BGA type memories 13 may be tested. In this manner, according to the present modification, the electronic components may be tested before the flexible substrate 15 is disposed on the printed circuit board 11.
Next, a description is given of a printed circuit board unit according to a second embodiment of the present invention.
A flexible substrate 48 disposed on the printed circuit board unit 10C according to the present embodiment includes a fixed part 48A fixed to the back side 11B of the printed circuit board 11, and the extending part 48B, which extends outward from the fixed part 48A and which is separated from the printed circuit board 11.
The flexible substrate 48 serving as a slave substrate in the present embodiment is a resin film having insulating properties and flexibility such as a polyimide film. The same third lands 38 and fourth lands 39 and 40 as those illustrated in
The fourth lands 40 are formed on the back side of the fixed part 48A, and the bypass capacitors 14 are mounted on these fourth lands 40. The third lands 38 for connecting to the printed circuit board 11 are formed on the front side of the fixed part 48A. The third lands 38 are soldered to the second lands 37 formed on the back side 11B of the printed circuit board 11 for connecting to the flexible substrate 15. Accordingly, the flexible substrate 48 is fixed to the printed circuit board 11. The method of connecting the second lands 37 formed on the printed circuit board 11 to the third lands 38 formed on the flexible substrate 15 is not limited to soldering; a pressure bonding method or an ACF (Anisotropic Conductive Film) connecting method are also applicable.
The description of the printed circuit board unit 10C continues below. On the front side of the extending part 48B, the bypass capacitor 14 or the BGA type memory 13 is mounted. Meanwhile, on the back side of the extending part 48B, the BGA type memory 13 is mounted. Accordingly, as illustrated in
In the printed circuit board unit 10C according to the present embodiment, the extending parts 48B of the flexible substrate 48 are not fixed to the printed circuit board 11. Rather, the extending parts 48B are separated from the back side 11B of the printed circuit board 11. Accordingly, an empty space 55 is formed between the back side 11B of the printed circuit board 11 and the extending part 48B of the flexible substrate 48 (see
In the printed circuit board unit 10A according to the first embodiment, the openings 19 are formed in the flexible substrate 15, and the bypass capacitors 14 mounted on the printed circuit board 11 are positioned in the openings 19, so that the components are densely mounted (see
Meanwhile, in the printed circuit board unit 10C according to the second embodiment, the extending part 48B is separated from the printed circuit board 11. Therefore, the electric components may be provided on both the front side and the back side of the extending part 48B, and also on the back side 11B of the printed circuit board 11 facing the extending part 48B. In the example illustrated in
As described above, in the printed circuit board unit 10C according to the present embodiment, the electronic components are mounted even more efficiently compared to the case of the printed circuit board unit 10A.
A fixed part 50A of the rigid flexible substrate 50 is fixed to the printed circuit board 11. On extending parts 50B of the rigid flexible substrate 50, electronic components such as the BGA type memory 13 may be mounted. The fixed part 50A and the extending parts 50B are made of a hard and highly rigid material such as glass epoxy. A flexible part 50C for connecting the fixed part 50A and each of the extending parts 50B is formed with a flexible substrate. The fixed part 50A and the extending part 50B are highly rigid, and therefore the process of solder-mounting the fixed part 50A to the printed circuit board 11 and the process of mounting electronic components such as the BGA type memory 13 may be reliably performed.
Meanwhile, a flexible substrate 51 illustrated in
Furthermore, a square part including the tips of the incisions 51C as apexes (the part surrounded by dotted lines in
A flexible substrate 52 illustrated in
According to an embodiment of the present invention, in a printed circuit board unit, a flexible substrate is provided on the back side of a printed circuit board in a region corresponding to where an integrated circuit is mounted on the front side of the printed circuit board. By using the flexible substrate, components may be mounted efficiently.
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
Number | Date | Country | Kind |
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2009-164034 | Jul 2009 | JP | national |